The IEEE 1003.1e draft 17 ("POSIX.1e") describes a set of 28
functions. These are grouped into three groups, based on their
portability: 
    - first group, the most portable one. All systems which claim to
      support POSIX.1e should implement these: 
     acl_delete_def_file(3), acl_dup(3), acl_free(3), acl_from_text(3),
     acl_get_fd(3), acl_get_file(3), acl_init(3), acl_set_fd(3),
     acl_set_file(3), acl_to_text(3), acl_valid(3)

    - second group, containing the rest of the POSIX ACL functions. Systems
      which claim to fully implement POSIX.1e should implement these:
     acl_add_perm(3), acl_calc_mask(3), acl_clear_perms(3), acl_copy_entry(3),
     acl_copy_ext(3), acl_copy_int(3), acl_create_entry(3),
     acl_delete_entry(3), acl_delete_perm(3), acl_get_entry(3),
     acl_get_permset(3), acl_get_qualifier(3), acl_get_tag_type(3),
     acl_set_permset(3), acl_set_qualifier(3), acl_set_tag_type(3),
     acl_size(3)

    - third group, containing extra functions implemented by each OS. These
      are non-portable version. Both Linux and FreeBSD implement some extra 
      function.

Thus we have the level of compliance. Depending on whether the system
library support the second group, you get some extra methods for the
ACL object.

Internal structure
    The POSIX draft has the following stuff (correct me if I'm wrong):
        - an ACL is denoted by acl_t
        - an ACL contains many acl_entry_t, these are the individual entries
          in the list; they always! belong to an acl_t
        - each entry_t has a qualifier (think uid_t or gid_t), whose type is 
          denoted by the acl_tag_t type, and an acl_permset_t
        - the acl_permset_t can contain acl_perm_t value (ACL_READ, ACL_WRITE,
          ACL_EXECUTE, ACL_ADD, ACL_DELETE, ...)
        - function to manipulate all these, and functions to manipulate files
